What is up with my rear drum brakes? Acting warped even freshly machined

I have 11x2.5 inch rear drums on my 1967 Coronet wagon and I can't figure out why they aren't running true.

Before taking them off to be machined & reinstalling them with new shoes, either wheel would catch on a higher spot on the drum when I tried to adjust them out, and you could feel the rear end pulsing under braking. They seem to catch in one spot on the drum, not two areas 180 degrees from each other

A couple years ago I took them off, had them turned, and threw them back on, still had the problem, but said "eh" as I wasn't driving the car like I do now. Back then the guy who helped me said it didn't even look like they'd been turned before.

This time, I have a GM Metric disc swap up front. The drums were machined to 11.059 and 11.060 by the same automotive machine shop here (not parts store) and immediately after bolting the wheels back on – car in the air, lugs hand-tight, both drums still dead cold and never used - the drums still don't drag evenly and in fact act almost the same as pre-machined, again.

I can't adjust the shoes out for slight drag because either drum catches on a certain spot as if it's warped. So right now I have slight drag across the area the shoes want to catch on and then they're too far in for the rest of the rotation. This still manifests if I have the wheels off (the wheels aren't warping otherwise-straight drums).

The pulsation manifests in normal braking, but for whatever reason (both 4 wheel drums and drum/disc) it never really gets that bad. The rear end will practically hop up and down if I bury the parking brake from a moderate roll, but repeated bedding-in stops didn't make anything spooky happen.

What could be out of whack in my rear end that would cause this? Or could the drums somehow just be off in a dimension that wouldn't appear on a lathe? Ordinarily I'd just throw new parts at this, but I can't find 11x2.5 rear drums for a decent price anywhere.

Bent axle???

I would wonder about the drums being off center as you mentioned.

I once had a wheel balance issue that would NOT go away. I tried everything. I even suggested the drums were out of balance, a machinist assured me that never happens... guess what? The damn drums where so out of balance they created a wheel imbalance.

I bought some new (Canadian made) drums, all was well (I think it took me at least a year to track that down).

Shoes installed properly?

This happens on both sides, not just one. The center of the drums fit over a taper on the end of the axle. And this is dead cold, not just after they're hot.

Is there some way I could distinguish between crap bearings & bent axles without pulling the axles?

turboswede wrote: Shoes installed properly?

Yes, it's pretty hard not to on this car. How could that cause static & dynamic warping-like behavior though?

I agree with Beaker, check for bent axles.

Something is bent or was made out of true.

I've seen a few drums made out of true, meaning that the center bore isn't true relative to the machined surface of the drum. A good machinist should be able to catch that and fix it easily. Its surprisingly common on large industrial drum brakes.

Its also possible that the axle end flanges aren't true relative to the axle. You could probably use a dial indicator to check that. I doubt that is the problem though.

Have you checked that the backing plates are in good condition? Bent backing plates can cause a lot of braking problems.

Ive never had drum brakes that didn't have some spot that contacted first. These were mainly GM pickups, but also some GM and Ford cars. None produced noticeable pulsation unless the adjusters were taken up to much.

I was taught to adjust drum brakes to the point that the shoes just touched the high spot when the wheel was rotated by hand. When adjusting them, I rotate the wheel until I can hear the shoes scuff in the "high" spot, but not drag.

I was just gonna say "off center." I had a similar problem on my Samurai with drums where the mounting holes had become ovalized, allowing the drum to shift around between the wheel and the hub.

Secret_Chimp wrote: Is there some way I could distinguish between crap bearings & bent axles without pulling the axles?

Bad bearings would let the axle flange move around if you tried grabbing it and shaking it. An off-center axle flange would only show up if you rotated it while checking with a dial indicator - I'd put the indicator on the center register circle that locates the center of the drum. If that checks out, I'd next put the drum on and check the outside of the drum with the dial indicator, and see if the center hole in the drum is off-center.

Also, it's a Mopar - pulling the axle is going to just mean undoing four or five bolts on the brake flange and pulling that sucker out. No C-clips inside the diff to complicate things.

check for run out.

Something weird I saw a long time ago: drums which had been machined too fast. The finish was more like a screw thread than the smooth finish needed. This showed as a 'hands clapping' kind of noise when braking (the shoes were popping in and out rapidly) and it also had a vibration from the rear.

Curmudgeon wrote: Something weird I saw a long time ago: drums which had been machined too fast. The finish was more like a screw thread than the smooth finish needed. This showed as a 'hands clapping' kind of noise when braking (the shoes were popping in and out rapidly) and it also had a vibration from the rear.

When I was drag racing we did this on purpose to help slow the car faster after the traps. Eat shoes in a summer but man did it stop.

I'm betting your drums junk. you didn't say what year but some early mopars have extended knurls on the studs and they pass through the drums thus the drum is lug centric not hub centric as is the norm. Also the drum flange face is often bent prying off the drums causing an axial run out if the drum is held on the bent flange to machine the shoe surface is machined off perpendicular, when the wheel is tightened the wheel pushes the drum flange to the axle flange and you now have an odd shoe face.

how much are new drums?

Well I put it in the air today, and with the wheels on and spinning in neutral (some mild internal pressure thing allows this to happen) the left side is visibly wobbly. The right side isn't too bad to the eye, but it still hangs up in a single spot through one rotation (not to mention the bearing on that side really sounds like junk). I need the car mobile for a couple more days but I presume I'll find bent axles over the weekend. At least new axle & bearing sets are only like $300. But here I was hoping to never smell gear oil from this thing...

See you are never wrong if Bill the cat agrees with you

$300 for axles and bearings. Seems cheap enough for peace of mind

Time for a disc brake upgrade?

Gearheadotaku wrote: Time for a disc brake upgrade?

8.8 LSD and disc brakes from an Exploder V8 sounds like the plan IMHO

Secret_Chimp wrote: Well I put it in the air today, and with the wheels on and spinning in neutral (some mild internal pressure thing allows this to happen) the left side is visibly wobbly. The right side isn't too bad to the eye, but it still hangs up in a single spot through one rotation (not to mention the bearing on that side really sounds like junk). I need the car mobile for a couple more days but I presume I'll find bent axles over the weekend. At least new axle & bearing sets are only like $300. But here I was hoping to never smell gear oil from this thing...

You don't end up smelling too much of it; unless you've got a very unusual axle swap on that, you can just pull the axles from the end and not have to open the differential cover.

Fun developments! I knocked my left drum off and the axle flange itself runs dead true. I put the drum back on backwards to see how runny-outy it was:

http://www.youtube.com/watch?v=D8NCBcsbNCA

http://www.youtube.com/watch?v=buVdvrEs0cQ

Here you can see how the axle isn't contributing to the wobble (I know sideways, sorry):

http://www.youtube.com/watch?v=MS5FoI0OV8Q

Sooooo... bonus points for everyone who guessed the drums vs the axles.

Any tips on where to look for 66-74 11x2.5 rear drums? I can't even find them indexed in most places I usually shop (Summit, RockAuto, Jegs, Amazon). I've only located one vendor – they don't offer actual part numbers for cross-reference and they want $120 apiece: http://www.jimsautoparts.com/brake.htm

Unless there's some miracle chance of machining this out?

You should really try all that again with a dial indicator. A visually imperceptible runout in an axle would translate to that much runout on the drum. Furthermore any runout measurement taken off the drum is only valid with it mounted on its actual mounting face(you'd need to remove the backing plate). Step 1 would be to check runout on both the axle flange face and center with a dial indicator.

Fair enough... I'll make that trip to Harbor Freight I've been putting off tomorrow

Fun developments! My left axle barely had .005" of runout on the flange face, and that may have been surface schmutz I couldn't get off all the way. The drum runout (installed normally with some washers to shore against the lugs) was much larger at .038. The right side axle was more like .011" at the flange, probably a bit less if I took more time to clean it off.

At this point I went to O'Reilly's and ordered a set of drums speced for Challengers (slightly different in specs but not where it counts). If they run true, yay, if not... boo